It is known that various pollutant substances (ammoniacal nitrogen, carbon-based substances, etc.) contained in effluents, in particular in water, can be removed biologically. Some of the reactions--aerobic thus used are carried out by means of atmospheric oxygen and they thus require an external supply of oxygen in as much as this oxygen is not already present in the water to be treated.
Many processes and devices are known which make it possible to carry out such a biological removal in the presence of oxygen. Thus, with this aim, reactors, filters or biofilters are used.
Reactors simultaneously carry out the supplying with oxygen and the provision of physical supports for the growth and maintenance of the bacteria.
Filters use oxygen already present in the water and their filtering material serves as a support for the growth and maintenance of the bacteria. In addition to the biological reaction, filters ensure the usual functions of retention of the matter in suspension, followed by its removal during washing.
Biofilters combine the functions carried out by reactors and filters.
Thus, the production of biofilters requires the combination, in the same chamber, of all the functions required for the removal of pollutant substances biologically: bacteria-supporting functions, aerating functions and washing functions. It is seen that such a combination can lead to a complex and thus expensive production, as regards both the manufacturing costs and the operating and maintenance costs. Moreover, the internal aeration used in biofilters leads to stirring of the bed of granular material and this stirring can be detrimental to the formation of the biomass on the granular material which serves as a support for it. The internal aeration can also be detrimental, for the same reasons, to the quality of the filtered water.